Metamaterials are generally understood to be artificially synthesized materials that are typically characterized by a repeating pattern of structural elements that have characteristic lengths on the order of less than the wavelength of the waves that they are meant to impact. For example, ‘photonic metamaterials’ (also known as ‘optical metamaterials’), which are meant to control the propagation of visible light, include structural elements that have characteristic lengths on the order of nanometers—by contrast, the wavelength of visible light is on the order of hundreds of nanometers. Much research has been devoted to developing such materials that have highly counterintuitive, but practical, optical characteristics—for example, metamaterials having negative indices of refraction have been developed and are the subject of much study.
‘Metasurfaces’ can be thought of as two-dimensional metamaterials insofar as they are characterized by a repeating pattern of subwavelength structures, and they can offer many of the same advantages as metamaterials. Indeed, metasurfaces can even be advantageous relative to metamaterials in many respects. For example, metasurfaces can be made to more efficiently transmit light as compared to metamaterials.